A riddle assembly, be it of the single or multiple deck type, must be of relatively large surface area if it is to be used commercially with a high degree of efficiency. Use of several small-capacity screening arrangements is expensive and generally wasteful.
Certain design problems are encountered when it is attempted to build large-capacity screening devices. It has been found virtually impossible to build such a device having a width greater than 3 meters. Such devices, become with increased size, so massive and heavy that shaking or vibrating the screens requires an enormous drive and the resonant frequency of the large screen support approaches the shaking frequency and leads to recurrent breakdowns.
In one known riddle of extended width the sieve is supported on at least two longitudinal and symmetrically aligned carrying members interconnected by multispan cross beams. In such a device the riddle or sieve is vibrated by means of a drive affixed directly to the longitudinal carrying members. This riddle rests on elastic elements on the supporting structure.
A fundamental feature which makes this described construction different from conventional constructions is the outer spans which are the elements extending the riddle width. These outer spans are made in tubular form with a uniform section, selected in proportion to the value of maximum bending moments.
Such construction does not reduce the weight of the installation by much, this reduction being very important in the case of riddles of considerable width which are vibrated by means of a drive whose size and power demand depend mainly on the mass of the entire system.
The middle and outer spans in such a large-width riddle assembly are usually the same. Such an arrangement facilitates replacing of sieves in separate sections.
Mounting of riddle vibrators either directly on a carrying box-section member or above this member divides the screening surface into independent sections parallel to the direction of motion of the sieved material. This offers an additional difficulty for the sieving process since a uniform distribution of the material over the entire sieve area becomes impossible, thus reducing the efficiency of the process.